The setting of safe and reliable anchors is one of the most fundamental skills in climbing. Yet few concrete rules-of-thumb exist for determining what is enough protection. Abstract ideas, intuition, and even superstition often stand in lieu of hard numbers.
So, what makes an anchor?
How many points of protection are required?
How strong does an anchor need to be?
First, let’s consider some numbers. According to Taupin & Verdier in the French treatise Amenagement et Equipment d-un Site Naturel d’Escalade, the routine forces in climbing are: 2,600 pounds for lead climbing falls and 1,000 pounds for top roping falls. Hopefully, these numbers reinforce in your mind the importance of building strong anchors...you are going to put many hundreds of pounds of force on your anchors.
Fortunately, the greatest possible force that you can put on an anchor is limited.
To back up a step, it is important to realize that climbing ropes stretch. When they stretch, climbing ropes absorb energy. When a climber falls, some of the energy created in the fall gets translated to the anchor (and to the climber) while some of the energy gets absorbed by the rope as it stretches.
The International Mountaineering & Climbing Federation (also known as the UIAA) provides standards for how much energy a climbing rope should absorb. The UIAA-assumed maximum force for UIAA-certified ropes is 12kN (about 2,700 pounds) at the tie-in point and 24kN (about 5,400 pounds) at the top anchor.
In other words, the maximum force that you can apply to an anchor using a climbing rope is 24kN. Admittedly, heavier climbers apply more force than light climbers and the UIAA test weight is only 176 pounds. Additionally, falling repeatedly over a short period of time reduces the elasticity of the rope thereby increasing the force on an anchor with each subsequent fall. Even given these exceptions, 24kN is a very useful number.
My standard for anchor building is this: the best possible anchor is the one that will hold the worst possible fall. Build 24kN anchors and it is reasonable to believe that you are not going to break those anchors using a climbing rope.
How do you get to 24kN? How do you know how much force a camming device or a bolt or a natural anchor will hold? Ahh...that's a whole 'nother conversation.